The present invention extends the teachings of my copending application for patent, Ser. No. 947,344, filed Oct. 2, 1978, which is incorporated herein by reference.
It is well known in the art that primary recovery of crude oil from underground reservoirs is enhanced in situations where there is a high degree of dissolved gases within the oil in place. Two particularly useful gases for this purpose are natural gas (methane) and carbon dioxide, both of which are highly soluble in crude oil. These gases aid in recovery by causing reduced viscosity of the oil thereby facilitating its movement to the well bore. Carbon dioxide can play a unique role in petroleum production by extracting lighter fractions of the oil. Hydrogen, although considerably less soluble in oil than methane and carbon dioxide, also has several unique features that can be brought to bear on improving the recovery of petroleum.
A rather considerable amount of crude oil has been discovered in the United States within the last century. An oil reservoir generally is quite non-uniform in character, with wide variations in porosity, permeability, petroleum content, water content and the like from place to place through out the reservoir. It therefore follows that difficulties can be expected in displacing oil in place to a collection point such as a well bore. With over 100 years of experience in oil production techniques, overall oil recovery still remains a relatively small percentage, in the order of 30%, of the original oil in place. Thus there remains in known oil fields, after economic depletion of the reservoir, approximately twice as much oil trapped in the reservoir as was produced during the flush and lean phases of production. This remaining oil is of growing significance as the supplies of available oil change from surplus to shortage as compared to crude oil demand.
The reason substantial quantities of oil remain in place after an oil field has been abandoned is that the cost of producing the oil exceeded its profitable selling price at the time production was terminated. It is possible, of course, to recover virtually all of the oil from a reservoir if cost of production is no object. In the extreme case the petroleum reservoir can be grubbed out with the contents removed to the surface of the earth for separation of the oil from the host rock, the connate water and the like. Such an approach, in addition to being costly also introduces environmental problems of considerable magnitude.
Thus the practical approach is toward more effective recovery of the petroleum from the reservoir by displacement of the oil in place to a series of well bores for recovery. Several schemes of displacement have been developed that increase recovery of crude oil over that which is attainable from natural reservoir drive. Water, for example, can be injected into the reservoir to provide artificial drive which sweeps oil toward production wells. In some cases, particularly when the crude oil is of relatively low API gravity, a portion of the oil in place can be burned by injecting high pressure air with the resultant cracking and distillation of nearby oil into more mobile fractions, which also adds heat to reduce viscosity of unburned oil as well as adding differential pressure to the reservoir.
In the case of ordinary water injection, rarely will oil recoveries exceed 50% of the oil in place. There are two principal problems mitigating against higher recovery: tight formations in portions of the reservoir that exclude penetration of the water and the mismatch between the viscosities of water and oil. As a practical matter there is not much that can be done about opening tight portions of the reservoir so that water flooding can proceed uniformly through this portion of the pay zone. The mismatch in viscosities is much easier to correct by adding soluble polymers to the water in what is known as the micellar slug technique. The reservoir is first treated by injecting detergents to lower the surface tension of the oil, followed by a slug of thickened water containing dissolved polymers, with a final slug of ordinary water being injected to complete the sequence.
In the case of fire floods a portion of the crude oil is burned, generally in the order of 15% of the oil in place, which precludes that portion of the oil from being recovered. Also there is the troublesome problem of keeping the underground fire going in the planned manner of reservoir sweep. In order for a fire to propagate it is necessary to provide injected air in proper proportions to burn the fuel within its limits of flammability. When air volumes are provided outside the limits of flammability, the fire will be extinguished. An improvement can be made, however, by adding a fuel to the reservoir that has significantly wider limits of flammability, as will be more fully disclosed hereinafter.
With the worldwide tightening of crude oil supplies with respect to demand, crude oil prices have risen dramatically and are expected to continue rising as supplies become tighter. This trend of escallating prices changes the economics of crude oil production, making residual oil of previously "depleted" oil fields an attractive candidate for future production. Higher prices also encourage the routine use of production techniques that heretofore were unattractive despite the fact that overall recoveries could be improved. There seems to be no question but what a substantial amount of increased supply of energy required in the United States in the future years will come from a single reliable source; known oil fields using more expensive but effective enhanced recovery techniques. It is an object of the present invention to teach techniques of improved crude oil recovery.